Torque limiting tools are extensively used to tighten a fastener to a specific torque. Such tools are extensively used during surgical procedures, such as an orthopedic surgical procedure. For example, a torque limiting tool may be used to tighten a fastener that is used to secure an orthopedic implant or bone plate. As such, it is often important that the fastener is tightened to a specific torque. Over tightening a fastener could result in damage to the orthopedic implant or bone plate. Likewise, a fastener that is not adequately tightened, may result in undesirable movement of the implant or bone plate within the patient. Such movement of the orthopedic implant or bone plate may be adverse to a patient as the implant may move to a position that is not efficacious to the patient.
Orthopedic bone plates play a critical role in the healing process of broken bones. Once a bone has been fragmented, it is often ideal for the broken bone fragments to be joined back together under compression to promote improved healing. The bone plate is a critical device that is used as a stabilizing bar that bridges the gap in bringing the bone fragments together.
During surgery, a bone plate is inserted next to the fragmented bone of a patient. Various compression and locking screws are typically placed through the bone plate to thus anchor the bone fragments together. They are then anchored into each of the bone fragments and tightened, pulling the bone fragments together under a compression load. Once the compression screws are set in place, locking bone screws are inserted through the bone plate and anchored into the fragmented bone. The locking bone screws in conjunction with the bone plate secure the bone fragments together and ensure that they do not move. Therefore, it is important that the compression and locking screws are tightened to a specific torque to ensure that the bone plate is securely in place, thus minimizing the possibility of causing further damage.
The medical industry has made use of both reusable and single use torque limiting tools. As previously mentioned, it is important that the torque limiting tool, regardless of whether it is designed to be reusable or for single use, be capable of imparting a precise amount of torque. Reusable torque limiting tools require frequent recalibration to ensure a precise amount of torque is imparted by the tool. Recalibration is a cumbersome process that must be performed routinely to ensure the tool operates correctly.
Single use torque limiting tools are easy to use and are a reliable alternative to a reusable tool. A single use torque limiting tool may be packaged with each implant and be specifically designed according to the implant's specifications. Once the tool has been used, it can be discarded, thus ensuring the torque limiting tool will impart a precise amount of torque that is required to secure the implant.
The present invention provides a single use torque limiting tool having an improved torque limiting mechanism that ensures the application of a precise amount of torque. Unlike prior art torque limiting tools, such as the torque limiting device disclosed by Nino et al. in U.S. patent application publication number 2009/0293687, the torque limiting tool of the present invention does not comprise a bias member or a plurality of washers to establish a torque limit. In contrast, the torque limiting mechanism of the present invention comprises two opposing gears, each of which has a plurality of outwardly extending prongs having an angular orientation with opposing ramp surfaces. Torque is thus transferred between the gears when at least two of the opposing prongs are in physical contact therebetween.
More specifically, the prongs of the gears of the torque limiting mechanism of the present invention are positioned such that the respective prong ramp surfaces are positioned in opposition to each other. Torque is transferred between the gears when the opposing prongs physically contact therebetween. However, once a maximum amount of torque is exceeded, the prongs of the opposing gears disengage. Specifically, either of the prongs of the opposing gears flex towards the respective gear surface, thereby enabling the opposing prongs to slide off the ramp surface, thus disengaging the gears and preventing torque transfer between the gears.
Therefore, in addition to the structural differences between the torque limiting device of Nino and other prior art devices, the amount of torque imparted by the present invention is limited by the mechanical properties of the prong. More specifically, in contrast to prior art devices, the amount of torque imparted by the tool of the present invention is limited by the ability of the respective prongs to resist mechanical flexure.